A YBa.sub.2 Cu.sub.3 O.sub.7-x type (123 phase type) superconductor prepared by a QMG (quench and melt growth) method, that is, a kind of a melt method, apparently has a critical current density (Jc) of 10.sup.4 A/cm.sup.2 or more under conditions of 77K and 1T which suffices for practical use (see "New Superconducting Materials Forum News", No. 10 (1988), p. 15). Studies have been made also on an increase in the size of the QMG material and a combination of RE elements (see "Physica C", 162-164 (1989), pp. 1217-1218 or Preprints of the 50th Symposium of the Japan Society of Applied Physics, Autumn, 1989, 29a-P-10). In these methods, an oxide superconductor material containing Y element alone or various RE elements is unidirectionally grown in a temperature gradient to increase the size of a crystal.
In a polycrystalline structure, a grain boundary acts as a weak link to decrease superconducting properties. The above-mentioned study on the increase in the size of the crystal has been made for the purpose of solving this problem.
The above-mentioned prior art technique, however, can provide only a single crystal material having a size of 0.3 cm.sup.3 at the largest, and it was very difficult to unidirectionally grow the 123 phase structure to form a large single crystal.
That is, the prior art could not solve problems regarding the means for forming a nucleus of a crystal, a method of regulating the growth of a crystal, etc.
An object of the present invention is to provide a material comprising a REBa.sub.2 Cu.sub.3 O.sub.7-x phase in a large-size single crystal form (hereinafter referred to as "123 phase") in an oxide superconductor comprising a composite oxide of at least two elements selected from the group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu (hereinafter referred to as "RE elements"), Ba and Cu.
Another object of the present invention is to provide a material having a structure such that a RE.sub.2 BaCuO.sub.5 phase (hereinafter referred to as "211 phase") having a very small grain diameter is dispersed in a large size 123 phase.